Method of manufacturing electric heating units



Feb. 19, 1935. c. c. ABBOTT 1,991,591

METHOD OF' MANUFACTURING ELEC'RIC HEATING UNITS Filed July 3.0, 1931 5 /65 Fign.

lrwverof: Charles C. Abo'bt,

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, supporting insulators. .vides a terminal structure having great electrical ratnied Feb. 19, 1935 l UNITED STATES.

METHOD OF MANUFACTURING ELECTRIC HEATING UNITS Charles C'. Abbott, Pittsfield, Mass., assignor to General Electric Company, corporation of New York Application July 30, 1931, Serial No.

(Cl. 20L-67) 2 Claims. l

My invention relates to electric heating units and has for its object the provision of an im-l proved heating unit of this character and method of making it.

My invention has special application to electric heating units in which a metallic sheath is provided, although it has application to various other types of electric heating units.

Inone of its aspects, my invention contemplates the provision of an electric heating unit of the sheathed type having a very strong construction both mechanically and eiectrically. My invention further contemplates the provision of simple, reliable and inexpensive connections between the resistance conductor and its terminals and between the terminals and the supply leads.

In accordance with my invention, I provide substantially solid insulating supporting members for the resistance conductor and terminals in vthe end portions of the sheath, the insulators being provided with apertures in which the terminals rest and through which they pass tothe exterior of the sheath. In order to increase the mechanical and electrical strength of the heating unit, insulating blocks are placed over the projecting portions of the terminals, these blocks being provided with apertures for receiving the terminals. In one form of my invention,l these latter insulators are provided with extensions surrounding the terminals and projecting into the ends of the sheath. These projections which extend for a substantial distance into the sheath are received in recesses provided for them in the This arrangement proand mechanical strength. In another form of my invention, I increase the electrical strength of the terminal structure, by arranging suitable insulating sleeves about the terminals where they emerge from the supporting insulators.

In order to effect a strong mechanical and good electrical connection between the resistance conductor and its terminals, I provide a hole through 'each terminal and countersink or otherwise provide suitable recesses in the ends of the terminal communicating with the hole. The resistance element is then inserted in the hole andl brazed to the terminal through the recesses,4 the latter being filled with the brazing material. This material effects a very good electrical and mechanical connection between the terminal and resistance conductor. If desired, and it is usually preferable to do so, a slot may be arranged in the terminal between its ends and communicating with the hole, the resistance element being brazed to the terminal through the slot.

The connections between the terminals and leads are effected by means of suitable connector members. Each connector is compressed on its lead to provideva strongmechanical and goodelectrical connection therewith; and preferably will be secured to its terminal by means of a threaded connection. To insure the electrical conriection between the terminal and connector I insert between them a material having a good electrical conductivity, such as a suitable brazing material, the connector being arranged so that this material also is in electrical contact with the resistance conductor and lead.

The connections between the resistance conductor and its terminals and between the terminals and the leads are claimed specifically in my copending application, Serial No. 628,693, iiled August 13, 1932, which is a division of this application and which is assigned to the same assignee as this invention.

For a more complete understanding of my invention, reference should be had to the accompanying drawing in which Fig. 1 is an elevation of an electric heating unit constructed in accordance with my invention, the heating unit being provided with a terminal at each end; Fig. 2 is a fragmentary view in section illustrating a portion of the finished electric heating unit of Fig.

1; Fig. 3 is a fragmentary view illustrating a portion of the heater of Fig. 1, and more particularly the connection between the terminal and lead connector; Fig. 4 is an elevation partly in section of the heater of Fig. 1 partially completed, parts being broken away; Fig. 5 is a sectional view taken through the line 5-'5 of Fig. 4 and looking in the direction of the arrows; Fig. 6 is an elevation in section illustrating the heater in a more advanced stage of its construction; Fig. rI is a plan view on an enlarged scale of a terminal used in the heater of Fig. 1 and formed in accordance with my invention; Fig. 8 is an elevation in sec tion of a c-onnector member used in my heating unit; Fig. 9 is an elevation of an electric heating unit formed in. accordance with my invention, this heater having its terminals at one end of the unit; Fig. 10 is an elevation in section of the heat- -ing unit of Fig. 9, portions being broken away;

Fig. 11 is an elevation in section of the heating unit of Fig. 9 partially completed, portions being broken away; and Fig. 12 is a sectional View taken along the line 12-12 of Fig. 10 and looking in the direction of the arrows.

Referring to Figs. 1 to 8 inclusive of the drawing, in one form of my invention the heating unit comprises a helical resistance conductor 10 mounted within a metallic sheath 11 and embedded in a suitable compacted, powdered, heat refractory insulating material 13, such as magnesim oxide. Terminals 14 and 15 are secured, to the ends of the resistance element. 'I'hese terminals are received in suitable supporting blocks of insulating material 16 and 17 secured in the end portions of the sheath, these insulators being `provided with substantially centrally arrangedYYY larger than the outside diameter of the terminal.

After the terminal and its supporting insulator have been assembled a sleeve 42 is placed on the projecting end of the terminal and suitable nuts 43 are screwed on to draw the insulator 16 tight up against the head piece 36.

In the other end of the sheath is placed a suitable plug-like loading member 44. This member, as shown, extends for some distance into the sheath and is provided with a flange 45 which rests on the end of the sheath to prevent the withdrawal of the member into the sheath. `This loading member is provided' with a central aperture 46 in which the terminal 15 is received, and with. a plurality of apertures 47 surrounding the central aperture. A suitable sleeve 50 is placed on the projecting end portion of the terminal 15 and a nut 51 is then threaded on the terminal to draw its enlarged portion 15a tightly up against a seat provided for it in the loading plug 44. This tensions the resistance element somewhat. A blind nut 52 is then threaded on the extreme end portion of the terminal, this nut serving to lock the first nut 51 and to prevent the magnesium oxide from filling the terminal threads during the loading operation so that thereafter the nuts may be easily removed.

The sheath is then indented at substantially its central portion so as to form a reduced retaining section for thewcentral portion of the resistance conductor. As clearly shown in Figs. 4 and 5, the sheath is indented so as to form a pair of flattened portions arranged substantially at right angles to each other. These flattened portions forma substantially square reduced section for the helical coil, as shown in Fig. 5. It will be understood that this reduced section serves to hold the conductor centrally of the sheath during the subsequent insulation loading and compacting operations.

The sheath is then ready for the loading operation. The sheath is now filled through the openings 47 provided in the loading plug with the heat refractory powdered insulatingI material 13. To accomplish this, the sheath is held upright in a special vibrating machine (not shown) with the end provided with the loading plug uppermost. As the sheath is illled with the insulating material through the openings 47, it is vibrated to shake the insulating material together and distribute it uniformly around the terminals and resistance element.

When the sheath has been illled, the nuts 51.

and 52 are removed and so also are the sleeve 50 and the loading plug 44. The supporting insulator 17 is then placed in the right hand end of the sheath in the space left by the plug 44. After the suppcrting insulator 17 is assembled, a head piece 36a, similar to the head piece 36, is assembled with the sheath and is secured by a spun-over portion 37a as clearly shown in Fig.

6. The insulator 17 is then secured by means of a sleeve 53 and nuts 54. It will be understood that the tensioned helical resistance conductor 10 will not withdraw its terminal 15 into the sheath when the plug 44 is removed because the magnesium oxide will be sufllciently compacted to hold the element and terminal in position for the reception `of the supporting insulator 17.

After the sheath has thus been loaded with the powdered insulating material I compact the insulating material about the terminals and resistance element in any suitable manner, but preferably by swaging the sheath. To effect this, the entire unit is passed through a swaging machine, the swaging operation reducing the diameter and increasing the length of the unit. Several passes through the swaging machine usually are necessary, and if desired the heating unit may be annealed 4between successive swaging operations in order to prevent the resistance element and sheath from becoming hard and brittle. -The square retaining section formed in the central portion of the sheath is removed during the swaging operations. It is to be noted that by reason of the fact that the end portions of the sheath have been reduced by previous operations, the central portions of sheath receive a greater amount of Aswaging in the main swaging operations, whereby the magnesium oxide is more perfectly compacted; the end portions of the sheath during these swaging operations are merely reduced suliciently to positively grip the insulating supports 16 and 17. As has been pointed out, the apertures provided in the head pieces 36 and 36a when assembled were considerably larger than the diameters of the terminals which were received in them;. this allows the head pieces to be reduced considerably during the swaging operation without allowing them to. grip the terminals. After the heating unit hasI been loaded with the magnesium oxide and has been reduced to compact the oxide, the end pcrtions of the sheath are threaded to provide for the threaded connections between the sheath and the protective casings 30.

Then the projecting portions of the head pieces 36 and 36a are cut oil alongthe line A--A as shown in Fig. 6. These severed portions may easily be withdrawn because, as has been pointed out, they do not grip the terminals after the swaging operation. This operation leaves the flangev portions of the head pieces wedgedin between the spun-over portions 37 and .37a and the supporting insulators 16 and 17. This operation also exposes the end portions of the supporting` insulators 16 and 17. These insulators are then provided with the recesses 25, as shown in Fig. 2, for the reception of the terminal insulators 22. The terminal insulators 22 are thenv assembled, their reduced portions 22a being received in the recesses 25. It will be understood that the insulators 22 will have been given the desired form at any suitable previous time.

The leads 28 are then attached to the heating unit. As shown in Fig. 8, each connector 26 used to connect the leads with the terminals is provided with an enlarged portion 26a having an aperture therethrough and a reduced portion 2Gb having an aperture therethrough, this second aperture being provided with an internal thread. The bared end portion of the conductor of the lead is inserted in the aperture provided in the enlarged portion 26a, and this portion is then reduced in diameter, as by swaging, so as to compress it on the conductor. The effect of this operation is to leave a head on the end of the conductor as is clearly shown in Fig. 3. This effectively prevents the withdrawal of the lead from the connector. After the connector has thus been secured to the lead, the connector is provided with a pair of apertures 56 (Figs. 2 and 3) communicating with the reduced threaded portion of the connector at points adjacent the bared end of the lead. If desired the insulation of the lead may be secured by means of a wire band 57 wrapped about the insulation adjacent the connector. The connectors are then threaded on their terminal members 14 and 15.

The connectors, the adjacent bared ends of the lead conductors and the brazing metal in the recesses 33 of the terminals lfl and l5 are brazen through the openings 56, the spaces between the lead conductors and the ends of the terminals being filled with the brazing metal, rlhis hraiiing metal, as before, preferably will be silver solder.

It will be observed that each connector has its end bearing on the terminal insulator 22 under-cut somewhat so as to provide a knife-edge to bear against the insulator. The purpose ci' this knife-edge is to prevent damage to the insulators in the event the connectors be screwed down too tightly on the terminals. In other words, if the connectors be screwed down too tightly the knife-edges will give way before the insulators and thus the latter 'will be protected.

After the leads have been attached, the casings 30 are screwed on the ends of the sheath.

The above heating unit has s; very strong ine-n chanical and electrical construction. The inm sulators 16 and 17 wedged tightly in the end por tions of the sheath effectively center and support the resistance conductor and .its terminals, and increase the electrical strength of the unit. rfhe provision of the terminal insulators 22 further increases the electrical strength of the unit. .And the connections which I have provided between the terminals and the resistance conductor are very strong mechanically and electrically. It will be observed that the electrically conducting metals arranged in the end recesses 33 and 34 of the terminals and also in the longitudinal ter.

minal slot 35 are in contact with a considerable area of the ends ofthe resistance conductor and also with the terminals so as to effect a very good electrical connection between these members. Moreover, it will be observed that the electrically conducting metal 58 arranged between the resistance conductor and leads insures the electrical connection between these members.

The heating unit shown in Fig. 9 is similar to the heating unit shown in Fig. l but has its terminals brought out at the same end of the unit rather than at opposite ends, as is the case of Fig. 1. 'I'his heating unit comprises a resistance element 60 supported on a suitable insulating core 61. The resistance element, as shown, preferably will be wound on the insulating core in the form of a double helix with the ends of the element at one end of the insulator and the midportion of the conductor formed in a loop 62 at the opposite end of the insulator.

The resistance conductor 60, together with its support 61, is supported in suitable cup-shaped insulating members 63 and 64 secured in the opposite end portions of the metallic sheath 65. These members will be formed of any suitable electrically insulating material, such as green lava. 'I'he left-hand end oi' this sheath is closed by a metallic washer-like member 66 while the terminals 67 and 68 project from the opposite end of the sheath. The insulating supporting member 64 is provided with a pair of apertures in which these terminals rest and through which they/pass to the' exterior of the sheath. These terminal members, as shown, are provided with` reduced portions which pass through the apertures provided in the support 64 and with enlarged portions which rest on the inner bottom surface of the support. h

The outer end portions of the terminal apertures provided in the insulator 64 are enlarged somewhat, as shown, and in these enlarged portions are placed suitable insulating sleeves 7G, formed from some suitable insulating material,

such as sheet mica. lit will be observed that these insulators entend .for a considerable disltanee downwardly :into the insulator Sil and project for a substantial distance from the outer sur-- :tace of the insulator. These sleeves may be .secured by means of threads 'il wrapped about them and formed from some suitalsle material, such as asbestos. Placed over the projecting portions ot these insulating sleeves and the projecting end portions of the terminals .is a terminal insuiati'n' 22, :formed from a suitable electrically insulating material, such as porcelain. This member, shown, is provided with a pair of apertures for receiving the projecting ends ci the terminals,

these apertures 'being enlarged to accommodate u' the mica sleeves 70. llt will be observed that the outer ends of the terminal apertures provided in the support te and the inner ends of the oper tures provided in the terminal insulator l2 are countersunk or otherwise enlarged to provide an :annular chamber for the threads l1 when the in sulator 'l2 is applied.

Suitable leads i3 are provided with connectors 74 which, as shown, have threaded connections with the terminals 67 and lill.

ln order to increase the electrical strength et the terminal end of the heating unit, e. .suitable barrier or sheet l5, formed of a suitable elec trically insulating material, such es mica, is interposed between the connectors 74. This member .has o substantially 'triangular shape, as shown in Figs. 10 and l2, the base oi' the triangle being received in e. suitable transverse slot T6 provided for it in the insulator 'l2 and the apex being secured against lateral displacement by means of a suitable asbestos thread 77 wrapped about the adjacent portions of the leads 73.

A suitable terminal protective casing 78 is threaded on the terminal end of the unit, as shown in Figs. 9 and l0.

The resistance conductor and those portions o1' the terminals within the sheath are sur rounded by a compacted, powdered, heat retractory, electrically insulating material 79, such as magnesium oxide.

In forming the heating element of Figs. 9 and 16, the resistance conductor is wound so as to form the double helix, previously described, in any suitable manner, as on a suitable arbor, and then is assembled with the insulating core 61.

The terminal members Pl and 68 are then .secured to the ends of the resistance conductor.

The terminals in this case are somewhat smaller in diameter than are the terminals of the unit of Figs. 1 and 2. by reason of the fact that 'there is less available space for the terminals in the single ended unit of Fig. 9. In view of this the end portions of the resistance conductor are reduced somewhat, as shown in Fig. 10, and the holes providedl for them in the terminals are shaped accordingly. This arrangement is advisable in order to strengthen the terminals at their threaded portions. The terminals are secured to the resistance element 60 in substantially the same manner as are the terminals 14 and 15 secured to the resistance element l0 of the heater of Fig. 1.

A fter the terminals have been secured, the conductor is assembled with its insulating supporting member 64. This supporting member with the assembled resistance conductor is then inserted in the metallic sheath. The sheath is prepared for the insertion of the resistance element and its supporting insulator 64 by reducing one of its ends, as for example, the right hand end as viewed in Fig. 11, and by providing in this end a head piece 80. This head piece is provided with a pair of apertures for receiving the two terminals 67 and 68, and each aperture is counterbored at its outer end as shown in Fig. 11. The head piece 80 is secured to the sheath by a spun-over portion 81.

Suitable metallic sleeves 82 with reduced ends to fit into the counterbores provided about the openings in the head piece are then arranged over the projecting ends of the terminals and are secured thereon by means of nuts A'83, these nuts being turned inwardly on the threaded portions of the terminals to draw the terminals and supporting members 64 snugly up against the head piece.

The supporting insulator 63 is then inserted in the other end of the sheath and over this insulator is inserted a loading washer 84 to which the loop 62, formed in the resistance coil, is secured by means of al suitable thread or wire 85 (Fig. 10). This thread or wire passes through a pair of apertures in the loading washer and is drawn tight and secured to the washer so as to place the resistance coil under 'a slight tension. It will be understood that the loading washer will be received in a suitable counterbore '88 provided in the sheath. If desired, the sheath may then be indented as was the sheath of the double ended unit of Figs. 1 to 8 so as to form a reduced retaining section (not shown) for the resistance conductor.

The loading washer 84 is provided with loading apertures 89. The sheath is now filled through these openings with the powdered magnesium oxide 79, the .unit being vibrated during this loading operation to distribute the insulating material and to compact it.

After the sheath has been loaded, the loading washer 84 is removed and the solid washer 66 (Fig. 10) is placed in the counterbore 88, the end of the sheath then being spun-over asindicated in Fig. 10 to secure the washer 66. Preferably this washer will be provided with a slight recess in its outer surface so that during the subsequent reducing or swaging operation the plate 66 will tend to bulge inwardly7 rather than outwardly. y

The unit is then swaged in a manner similar to the swaging operation described in connection with the unit of Fig. 1 so as to reduce the diameter and increase the length of the sheath, whereby the magnesium oxide is compacted to a hard dense mass about the resistance element and terminals.

The sheath is then provided with exterior threads on its terminal end for the reception of the protective casing 78.

After this, the mica insulating sleeves 70 are inserted in counterbores .provided for them in the supporting insulator 64 and are secured by means of the threads 71. Then the terminal insulator 72 is assembled.

The leads 73 are then attached to the terminals' by the connector members 74. These con-v nector members are of substantially the same construction as are the connector members 26 of the heater of Figs. 2 and Sand are secured to their leads in substantially the same manner. The connector members, as shown (Fig. 10), are threaded on their associated terminals 67 and 68. After the connectors are screwedon, they are brazed to the associated terminals by means of a suitable brazing material 90, such as silver solder,

through apertures 91, provided in the connectors. This brazing metal insures the electrical connection between the resistance element and its supply leads.

After the leads are thus attached, the triangular mica barrier or sheet 75 is assembled. It will be understood that the terminal insulator 72 will have been provided with the transverse slot 76 for receiving the base of the sheet 75; the apex of the sheet will then be secured by wrapping the asbestos thread 77 about the leads 73.

The terminal protective casing 78 isr then screwed on the sheath and is further secured by a set screw as shown.

The opposite end of the sheath may be closed by a suitable plate 92 arranged over the washer 66 as shown.

The terminal insulator 72, together with the y"insulating sleeves 70 and the insulating sheet 75 providing a very strong electrical terminal arrangement for the heater.

The electric heater disclosed in. Figs. 9, 10, 11

and 12 is described and claimed specifically in my copending application, Serial No. 725,040, filed May 11, 1934, which application is a division of the instant application, and which is assigned to l the same assignee as this application.

While I have shown particular embodiments of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modilications may be made, and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1. The method of making an electric heating Vunit which comprises securing a resistance element to a terminal, inserting said resistance element and terminal in a metallic sheath, supporting said terminal in a solid insulator and a metallic closure member arranged in one end of the sheath, filling the space around the resistance conductor with powdered heat refractory insulating material, reducing the diameter of said sheath to compact said insulating material and to secure said insulator, removing a portion of said metallic closure member to expose said insulator and then surrounding the projecting portion of said 'terminal with a solid insulator with said insulator in engagement with the exposed surface of said first insulator.

2. 'I'he method of making an electric heating unit which comprises securing a resistance conductor to a pair of terminals, reducing the end portions of a metallic sheath, inserting said resistance conductor and terminals in and lengthwise of said metallic sheath with the terminals at the reduced ends of said sheath and projecting from said sheath, supporting one of said terminals in an insulator and a metallic closure member arranged in the corresponding end of said sheath, lling said sheath through the opposite end with powdered insulating material, closing said end with a solid supporting insulator and metallic closure member, swaging said sheath to compact said insulating material, removing a portion of each of said metallic closure members to expose said supporting insulators, providing in said insulators recesses about the terminals, and then inserting in said recesses blocks of insulating material so as to surround the projecting terminal portions.

CHARLES C. ABBOTI. 

